Magnetic recording systems have been used as secondary storage in computer systems for many years. They have moderately fast access times and the capability to store massive amounts of data. As a result of advances in computer technology, there is a continuous demand for increases in the capacity of these systems (typically measured in terms of the total number of bits which can be stored), as well as for decreases in access time and cost. This demand has been met in the past, and will most likely continue to be met in the future, by increasing the density of data recording--that is, by increasing the number of bits which may be stored on a given amount of physical disk area. High recording densities allow large amounts of information to be stored in small volumes, which is particularly important, for example, in the case of portable computers. Moreover, higher densities enable both access times and cost to be reduced as well. It is therefore not surprising that recording density has been increasing rapidly in the last several decades. In fact, density has been approximately doubling every three years. At the present time, the maximum density which is commercially available in magnetic recording media (i.e., disk drives) is approaching 1 gigabit per square inch.
Increases in recording density are being achieved as a result of a combination of factors, such as improvements in magnetic materials, advances in read/write head designs, advances in servo technology that allow the magnetic head to be positioned with higher accuracy on the selected track (therefore reducing the necessary spacing between tracks), and finally, advances in the signal processing techniques used to decode the data from the signal generated by the read head. The need for more advanced signal processing techniques arises when density is increased because the electric pulses induced on the read head by adjacent bits tend to interfere and partially cancel each other, resulting in a decrease of the intensity of the detected signal. At the same time, the noise increases as a result of various effects, such as the interference from nearby (i.e., adjacent) tracks. These factors, among others, result in a rapid decrease of the signal to noise ratio (SNR) as recording density is increased. In some cases, advances in signal processing techniques can be used to compensate for this loss of SNR. For a description of certain signal processing techniques used in magnetic recording systems, see, for example, J. M. Cioffi, W. L. Abbott, H. K. Thapar, C. M. Melas, and K. D. Fisher, "Adaptive Equalization in Magnetic-Disk Storage Channels," IEEE Communications Magazine, February 1990, pp. 14-29, and P. H. Siegel and J. K. Wolf, "Modulation and Coding for Information Storage," IEEE Communications Magazine, December 1991, pp. 68-86. For a general discussion of magnetic recording systems, see, for example, R. W. Wood, "Magnetic Recording Systems," Proceedings of the IEEE, Vol. 74, No. 11, November 1986, pp. 1557-1569.
One of the signal processing techniques that has resulted in significant increases in density is the technique known as "Partial Response Maximum Likelihood" (PRML), described, for example, in H. Kobayashi and D. T. Tang, "Application of partial-response channel coding to magnetic recording systems," IBM Journal of Research and Development, Vol. 14, pp. 368-375, July 1970. In this technique, the discrete-time channel response is shaped by means of an adaptive filter referred to as the "equalizer" in such a way that its transfer function, expressed as a z-transform, is: EQU H(z)=1-z.sup.-2 ( 1)
This response is usually known as "Partial Response Class IV" (PR IV), and it is described, for example, in P. Kabal and S. Pasupathy, "Partial-response signaling," IEEE Transactions on Communications, Vol. COM-23, No. 9, pp. 921-934, September 1975. (Specifically, PRML is the result of combining PR IV with a technique known as "Maximum Likelihood Detection," described, for example, in G. D. Forney, "Maximum-Likelihood Sequence Estimation of Digital Sequences in the Presence of Intersymbol Interference," IEEE Transactions on Information Theory, Vol. IT-18, No. 3, May 1972, pp. 363-378.)
As pointed out above, increases in recording density rapidly degrade the SNR of the signal picked up by the read head. This makes it increasingly important to compensate for certain systematic sources of degradation of the SNR. One such source of degradation is known as intersymbol interference (ISI). This interference is the result of the detected pulses having a non-ideal shape which causes different pulses to interfere with each other. Specifically, ISI occurs because the detected pulses are not properly confined to the "time slot" (i.e., baud period) which is available to the reading of a single data symbol. (For example, a rectangular pulse confined to a given baud period will cause no ISI. However, it is not strictly necessary that the pulse be completely confined to its own time slot--Nyquist pulses, for example, if sampled at the correct instant, will also not cause ISI. Both confined rectangular pulses and Nyquist pulses are considered for purposes herein to be "properly confined" to the available time slot.)
When the detected pulses are not properly confined to the allocated time slot, neighboring pulses induced on the magnetic head by adjacent magnetic transitions on the disk surface will interfere with each other and will degrade the margin against random noise available to the detector. Of course, the width of the allocated time slot depends on the recording density. Therefore, increasing the density will reduce the size of the time slot associated with each pulse and will thereby generally increase ISI.
Intersymbol interference in a PRML receiver is partially compensated for by means of the adaptive filter referred to as the equalizer (see above). Specifically, this filter improves the shape of the detected pulses. More elaborate and complex equalizers do a better job of controlling ISI, and therefore result in improved SNR, which can, in turn, be translated to higher recording densities. Equalizers are usually implemented as adaptive transversal filters. In general, better equalization is achieved by increasing the number of taps of these filters. However, in practical implementations of magnetic recording systems, the number of taps is limited by cost and power dissipation.
The hardware used to process the output of a magnetic disk read head, commonly referred to as the "receiver," is typically implemented in a single VLSI chip in CMOS technology. This chip is commonly referred to as a "read-channel device." The data rates supported by present read-channel devices are in the 100 MHz (Megahertz) range and beyond. Since the power dissipation of CMOS VLSI chips is proportional to their speed of operation, high data rates imply high power dissipation, which is undesirable for numerous reasons including constraints in packaging technology. This, therefore, severely limits the complexity (e.g., number of taps) of the equalizer. As a result, it is impractical to completely compensate for ISI--in most implementations it is necessary to make compromises between performance and cost factors (including power dissipation which, indirectly, can be considered a cost factor).
One particular type of ISI that is common in magnetic recording channels is "precursor ISI." This specific type of interference is caused, for example, by a "leading undershoot" of a detected (positive) pulse. This undershoot commonly appears in the response of inductive read heads as a result of stray magnetic fields at the edges of the heads. ("Trailing undershoot" of a detected pulse also causes undesirable ISI known as "postcursor ISI." However, postcursor ISI can be compensated for using a conventional technique known as "decision-feedback equalization," familiar to those skilled in the art.) Although one way to compensate for precursor ISI would be to provide a sufficiently long transversal filter equalizer, this is usually impractical for the reasons discussed above. Without compensating for precursor ISI, the SNR may be degraded by about 2 dB (decibels) or more. Such a degradation translates into an undesirable loss of recording density of between approximately ten and fifteen percent, given the present state of the art.